Chapter 13 Exception Handling

Transcription

1 Chapter 13 Exception Handling 1

2 Motivations When a program runs into a runtime error, the program terminates abnormally. How can you handle the runtime error so that the program can continue to run or terminate gracefully? This is the subject we will introduce in this chapter. 2

3 Objectives To get an overview of exceptions and exception handling ( 13.2). To explore the advantages of using exception handling ( 13.3). To distinguish exception types: Error (fatal) vs. Exception (nonfatal), and checked vs. unchecked ( 13.4). To declare exceptions in a method header ( ). To throw exceptions in a method ( ). To write a try-catch block to handle exceptions ( ). To explain how an exception is propagated ( ). To use the finally clause in a try-catch block ( 13.6). To use exceptions only for unexpected errors ( 13.7). To rethrow exceptions in a catch block ( 13.8). To create chained exceptions ( 13.9). To define custom exception classes ( 13.10). 3

4 Exception-Handling Overview Show runtime error import java.util.scanner; public class Quotient { public static void main(string[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter two integers: "); int number1 = input.nextint(); int number2 = input.nextint(); System.out.println(number1 + " / " + number2 + " is " + (number1 / number2)); 4

5 Fix it using an if statement import java.util.scanner; public class QuotientWithIf { public static void main(string[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter two integers: "); int number1 = input.nextint(); int number2 = input.nextint(); if (number2!= 0) System.out.println(number1 + " / " + number2 + " is " + (number1 / number2)); else System.out.println("Divisor cannot be zero "); 5

6 What if the runtime error occurs in a called method? import java.util.scanner; public class QuotientWithException { public static void main(string[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter two integers: "); int number1 = input.nextint(); int number2 = input.nextint(); try { if (number2 == 0) throw new ArithmeticException("Divisor cannot be zero"); System.out.println(number1 + " / " + number2 + " is " + (number1 / number2)); catch (Exception ex) { System.out.println("Exception: an integer " + "cannot be divided by zero "); System.out.println("Execution continues..."); 6

7 Exception Advantages Now you see the advantages of using exception handling. It enables a method to throw an exception to its caller. Without this capability, a method must handle the exception or terminate the program. 7

8 import java.util.scanner; public class QuotientWithMethod { public static int quotient(int number1, int number2) { if (number2 == 0) throw new ArithmeticException("Divisor cannot be zero"); return number1 / number2; public static void main(string[] args) { Scanner input = new Scanner(System.in); // Prompt the user to enter two integers System.out.print("Enter two integers: "); int number1 = input.nextint(); int number2 = input.nextint(); try { int result = quotient(number1, number2); System.out.println(number1 + " / " + number2 + " is " + result); catch (Exception ex) { System.out.println("Exception: an integer " + "cannot be divided by zero "); System.out.println("Execution continues..."); 8

9 Handling InputMismatchException By handling InputMismatchException, your program will continuously read an input until it is correct. 9

10 import java.util.*; public class InputMismatchExceptionDemo { public static void main(string[] args) { Scanner input = new Scanner(System.in); boolean continueinput = true; do { try { System.out.print("Enter an integer: "); int number = input.nextint(); // Display the result System.out.println( "The number entered is " + number); continueinput = false; catch (InputMismatchException ex) { System.out.println("Try again. (" + "Incorrect input: an integer is required)"); input.nextline(); // discard input while (continueinput); 10

11 Exception Types ClassNotFoundException IOException ArithmeticException Exception NullPointerException RuntimeException IndexOutOfBoundsException Object Throwable Many more classes IllegalArgumentException Many more classes LinkageError Error VirtualMachineError Many more classes 11

12 System Errors ClassNotFoundException IOException ArithmeticException Exception NullPointerException RuntimeException IndexOutOfBoundsException Object Throwable Many more classes IllegalArgumentException System errors are thrown by JVM and represented in the Error class. The Error class describes internal system errors. Such errors rarely occur. If one does, there is little you can do beyond notifying the user and trying to terminate the program gracefully. Error LinkageError VirtualMachineError Many more classes Many more classes 12

13 Exceptions Exception describes errors caused by your program and external circumstances. These errors can be caught and handled by your program. Exception ClassNotFoundException IOException RuntimeException ArithmeticException NullPointerException IndexOutOfBoundsException Object Throwable Many more classes IllegalArgumentException Many more classes LinkageError Error VirtualMachineError Many more classes 13

14 Runtime Exceptions ClassNotFoundException IOException ArithmeticException Exception NullPointerException RuntimeException IndexOutOfBoundsException Object Throwable Many more classes IllegalArgumentException Many more classes Error LinkageError VirtualMachineError RuntimeException is caused by programming errors, such as bad casting, accessing an out-of-bounds array, and numeric errors. Many more classes 14

15 Checked Exceptions vs. Unchecked Exceptions RuntimeException, Error and their subclasses are known as unchecked exceptions. All other exceptions are known as checked exceptions, meaning that the compiler forces the programmer to check and deal with the exceptions. 15

16 Unchecked Exceptions In most cases, unchecked exceptions reflect programming logic errors that are not recoverable. For example, a NullPointerException is thrown if you access an object through a reference variable before an object is assigned to it; an IndexOutOfBoundsException is thrown if you access an element in an array outside the bounds of the array. These are the logic errors that should be corrected in the program. Unchecked exceptions can occur anywhere in the program. To avoid cumbersome overuse of try-catch blocks, Java does not mandate you to write code to catch unchecked exceptions. 16

17 Unchecked Exceptions ClassNotFoundException IOException ArithmeticException Exception NullPointerException RuntimeException IndexOutOfBoundsException Object Throwable Many more classes IllegalArgumentException Many more classes LinkageError Error VirtualMachineError Unchecked exception. Many more classes 17

18 Declaring, Throwing, and Catching Exceptions catch exception method1() { try { invoke method2; catch (Exception ex) { Process exception; method2() throws Exception { if (an error occurs) { throw new Exception(); declare exception throw exception 18

19 Declaring Exceptions Every method must state the types of checked exceptions it might throw. This is known as declaring exceptions. public void mymethod() throws IOException public void mymethod() throws IOException, OtherException 19

20 Throwing Exceptions When the program detects an error, the program can create an instance of an appropriate exception type and throw it. This is known as throwing an exception. Here is an example, throw new TheException(); TheException ex = new TheException(); throw ex; 20

21 Throwing Exceptions Example /** Set a new radius */ public void setradius(double newradius) throws IllegalArgumentException { if (newradius >= 0) radius = newradius; else throw new IllegalArgumentException( "Radius cannot be negative"); 21

22 Catching Exceptions try { statements; // Statements that may throw exceptions catch (Exception1 exvar1) { handler for exception1; catch (Exception2 exvar2) { handler for exception2;... catch (ExceptionN exvar3) { handler for exceptionn; 22

23 Catching Exceptions main method {... try {... invoke method1; statement1; catch (Exception1 ex1) { Process ex1; statement2; method1 {... try {... invoke method2; statement3; catch (Exception2 ex2) { Process ex2; statement4; method2 {... try {... invoke method3; statement5; catch (Exception3 ex3) { Process ex3; statement6; An exception is thrown in method3 Call Stack method3 method2 method2 method1 method1 method1 main method main method main method main method 23

24 Catch or Declare Checked Exceptions Java forces you to deal with checked exceptions. If a method declares a checked exception (i.e., an exception other than Error or RuntimeException), you must invoke it in a try-catch block or declare to throw the exception in the calling method. For example, suppose that method p1 invokes method p2 and p2 may throw a checked exception (e.g., IOException), you have to write the code as shown in (a) or (b). void p1() { try { p2(); catch (IOException ex) {... (a) void p1() throws IOException { p2(); (b) 24

25 Example: Declaring, Throwing, and Catching Exceptions Objective: This example demonstrates declaring, throwing, and catching exceptions by modifying the setradius method in the Circle class defined in Chapter 8. The new setradius method throws an exception if radius is negative. 25

26 public class TestCircleWithException { public static void main(string[] args) { try { CircleWithException c1 = new CircleWithException(5); CircleWithException c2 = new CircleWithException(-5); CircleWithException c3 = new CircleWithException(0); catch (IllegalArgumentException ex) { System.out.println(ex); System.out.println("Number of objects created: " + CircleWithException.getNumberOfObjects()); 26

27 public class CircleWithException { /** The radius of the circle */ private double radius; /** The number of the objects created */ private static int numberofobjects = 0; /** Construct a circle with radius 1 */ public CircleWithException() { this(1.0); /** Construct a circle with a specified radius */ public CircleWithException(double newradius) { setradius(newradius); numberofobjects++; /** Return radius */ public double getradius() { return radius; /** Set a new radius */ public void setradius(double newradius) throws IllegalArgumentException { if (newradius >= 0) radius = newradius; else throw new IllegalArgumentException( "Radius cannot be negative"); /** Return numberofobjects */ public static int getnumberofobjects() { return numberofobjects; /** Return the area of this circle */ public double findarea() { return radius * radius * ; 27

28 The finally Clause try { statements; catch(theexception ex) { handling ex; finally { finalstatements; 28

29 animation Trace a Program Execution try { statements; catch(theexception ex) { handling ex; finally { finalstatements; Next statement; Suppose no exceptions in the statements 29

30 animation Trace a Program Execution try { statements; catch(theexception ex) { handling ex; finally { finalstatements; The final block is always executed Next statement; 30

31 animation Trace a Program Execution try { statements; catch(theexception ex) { handling ex; finally { finalstatements; Next statement in the method is executed Next statement; 31

32 animation Trace a Program Execution try { statement1; statement2; statement3; catch(exception1 ex) { handling ex; finally { finalstatements; Suppose an exception of type Exception1 is thrown in statement2 Next statement; 32

33 animation Trace a Program Execution try { statement1; statement2; statement3; catch(exception1 ex) { handling ex; finally { finalstatements; The exception is handled. Next statement; 33

34 animation Trace a Program Execution try { statement1; statement2; statement3; catch(exception1 ex) { handling ex; finally { finalstatements; The final block is always executed. Next statement; 34

35 animation Trace a Program Execution try { statement1; statement2; statement3; catch(exception1 ex) { handling ex; finally { finalstatements; The next statement in the method is now executed. Next statement; 35

36 Cautions When Using Exceptions Exception handling separates error-handling code from normal programming tasks, thus making programs easier to read and to modify. Be aware, however, that exception handling usually requires more time and resources because it requires instantiating a new exception object, rolling back the call stack, and propagating the errors to the calling methods. 36

37 When to Throw Exceptions An exception occurs in a method. If you want the exception to be processed by its caller, you should create an exception object and throw it. If you can handle the exception in the method where it occurs, there is no need to throw it. 37

38 When to Use Exceptions When should you use the try-catch block in the code? You should use it to deal with unexpected error conditions. Do not use it to deal with simple, expected situations. For example, the following code try { System.out.println(refVar.toString()); catch (NullPointerException ex) { System.out.println("refVar is null"); 38

39 When to Use Exceptions is better to be replaced by if (refvar!= null) System.out.println(refVar.toString()); else System.out.println("refVar is null"); 39

40 Companion Website Assertions An assertion is a Java statement that enables you to assert an assumption about your program. An assertion contains a Boolean expression that should be true during program execution. Assertions can be used to assure program correctness and avoid logic errors. 40

41 Companion Website Declaring Assertions An assertion is declared using the new Java keyword assert in JDK 1.4 as follows: assert assertion; or assert assertion : detailmessage; where assertion is a Boolean expression and detailmessage is a primitive-type or an Object value. 41

42 Companion Website Executing Assertions When an assertion statement is executed, Java evaluates the assertion. If it is false, an AssertionError will be thrown. The AssertionError class has a no-arg constructor and seven overloaded single-argument constructors of type int, long, float, double, boolean, char, and Object. For the first assert statement with no detail message, the noarg constructor of AssertionError is used. For the second assert statement with a detail message, an appropriate AssertionError constructor is used to match the data type of the message. Since AssertionError is a subclass of Error, when an assertion becomes false, the program displays a message on the console and exits. 42

43 Companion Website Executing Assertions Example public class AssertionDemo { public static void main(string[] args) { int i; int sum = 0; for (i = 0; i < 10; i++) { sum += i; assert i == 10; assert sum > 10 && sum < 5 * 10 : "sum is " + sum; 43

44 Companion Website Using Exception Handling or Assertions Assertion should not be used to replace exception handling. Exception handling deals with unusual circumstances during program execution. Assertions are to assure the correctness of the program. Exception handling addresses robustness and assertion addresses correctness. Like exception handling, assertions are not used for normal tests, but for internal consistency and validity checks. Assertions are checked at runtime and can be turned on or off at startup time. 44

45 Companion Website Using Exception Handling or Assertions, cont. Do not use assertions for argument checking in public methods. Valid arguments that may be passed to a public method are considered to be part of the method s contract. The contract must always be obeyed whether assertions are enabled or disabled. For example, the following code should be rewritten using exception handling as shown in Lines in Circle.java in Listing public void setradius(double newradius) { assert newradius >= 0; radius = newradius; 45

46 Companion Website Using Exception Handling or Assertions, cont. Use assertions to reaffirm assumptions. This gives you more confidence to assure correctness of the program. A common use of assertions is to replace assumptions with assertions in the code. 46

47 Companion Website Using Exception Handling or Assertions, cont. Another good use of assertions is place assertions in a switch statement without a default case. For example, switch (month) { case 1:... ; break; case 2:... ; break;... case 12:... ; break; default: assert false : "Invalid month: " + month 47